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<title>The MPL Reference Manual: Tag Dispatched Metafunction</title>
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<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./metafunctions-concepts.html" class="navigation-link">Concepts</a> / <a href="./tag-dispatched-metafunction.html" class="navigation-link">Tag Dispatched Metafunction</a></td>
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<div class="section" id="tag-dispatched-metafunction">
<h1><a class="toc-backref" href="./metafunctions-concepts.html#id1516">Tag Dispatched Metafunction</a></h1>
<div class="section" id="id840">
<h3><a class="subsection-title" href="#summary" name="summary">Summary</a></h3>
<p>A <a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a> is a <a class="reference internal" href="./metafunction.html">Metafunction</a> that employs a
<em>tag dispatching</em> technique in its implementation to build an
infrastructure for easy overriding/extenstion of the metafunction's
behavior.</p>
</div>
<div class="section" id="notation">
<h3>Notation</h3>
<table border="1" class="docutils table" id="tag-metafunction">
<colgroup>
<col width="31%" />
<col width="69%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Symbol</th>
<th class="head">Legend</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><em>name</em></td>
<td>A placeholder token for the specific metafunction's name.</td>
</tr>
<tr><td><em>tag-metafunction</em></td>
<td>A placeholder token for the tag metafunction's name.</td>
</tr>
<tr><td><em>tag</em></td>
<td>A placeholder token for one of possible tag types
returned by the tag metafunction.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id841">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt; typename Tag &gt; struct <em>name</em>_impl;

template&lt;
      typename X
    <em>[, ...]</em>
    &gt;
struct <em>name</em>
    : <em>name</em>_impl&lt; typename <em>tag-<a href="./metafunction.html" class="identifier">metafunction</a></em>&lt;X&gt;::type &gt;
        ::template <a href="./apply.html" class="identifier">apply</a>&lt;X <em>[, ...]</em>&gt;
{
};

template&lt; typename Tag &gt; struct <em>name</em>_impl
{
    template&lt; typename X <em>[, ...]</em> &gt; struct <a href="./apply.html" class="identifier">apply</a>
    {
        // <em>default implementation</em>
    };
};

template&lt;&gt; struct <em>name</em>_impl&lt;<em>tag</em>&gt;
{
    template&lt; typename X <em>[, ...]</em> &gt; struct <a href="./apply.html" class="identifier">apply</a>
    {
        // <em>tag-specific implementation</em>
    };
};
</pre>
</div>
<div class="section" id="id842">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>The usual mechanism for overriding a metafunction's behavior is class
template specialization — given a library-defined metafunction <tt class="literal"><span class="pre">f</span></tt>,
it's possible to write a specialization of <tt class="literal"><span class="pre">f</span></tt> for a specific type
<tt class="literal"><span class="pre">user_type</span></tt> that would have the required semantics <a class="footnote-reference" href="#spec" id="id843">[4]</a>.</p>
<p>While this mechanism is always available, it's not always the most
convenient one, especially if it is desirable to specialize a
metafunction's behavior for a <em>family</em> of related types. A typical
example of it is numbered forms of sequence classes in MPL itself
(<tt class="literal"><span class="pre">list0</span></tt>, ..., <tt class="literal"><span class="pre">list50</span></tt>, et al.), and sequence classes in general.</p>
<p>A <a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a> is a concept name for an instance of
the metafunction implementation infrastructure being employed by the
library to make it easier for users and implementors to override the
behavior of library's metafunctions operating on families of specific
types.</p>
<p>The infrastructure is built on a variation of the technique commonly
known as <em>tag dispatching</em> (hence the concept name),
and involves three entities: a metafunction itself, an associated
tag-producing <a class="reference internal" href="./tag-dispatched-metafunction.html#tag-metafunction">tag metafunctions</a>, and the metafunction's
implementation, in the form of a <a class="reference internal" href="./metafunction-class.html">Metafunction Class</a> template
parametrized by a <tt class="literal"><span class="pre">Tag</span></tt> type parameter. The metafunction redirects
to its implementation class template by invoking its specialization
on a tag type produced by the tag metafunction with the original
metafunction's parameters.</p>
<table class="docutils footnote" frame="void" id="spec" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id843">[4]</a></td><td>Usually such user-defined specialization is still required
to preserve the <tt class="literal"><span class="pre">f</span></tt>'s original invariants and complexity requirements.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="id844">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/size.hpp" class="header">boost/mpl/size.hpp</a>&gt;

namespace user {

struct bitset_tag;

struct bitset0
{
    typedef bitset_tag tag;
    // ...
};

template< typename B0 > struct bitset1
{
    typedef bitset_tag tag;
    // ...
};

template< typename B0, <em>...,</em> typename B<em>n</em> &gt; struct bitset<em>n</em>
{
    typedef bitset_tag tag;
    // ...
};

} // namespace user

namespace boost { namespace mpl {
template&lt;&gt; struct size_impl&lt;user::bitset_tag&gt;
{
    template&lt; typename Bitset &gt; struct <a href="./apply.html" class="identifier">apply</a>
    {
        typedef typename Bitset::<a href="./size.html" class="identifier">size</a> type;
    };
};
}}
</pre>
</div>
<div class="section" id="id845">
<h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
<ul class="simple">
<li><a class="reference internal" href="./sequence-tag.html">sequence_tag</a></li>
</ul>
</div>
<div class="section" id="id846">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./metafunction.html">Metafunction</a>, <a class="reference internal" href="./metafunction-class.html">Metafunction Class</a>, <a class="reference internal" href="./numeric-metafunction.html">Numeric Metafunction</a></p>
<!-- Metafunctions/Concepts//Numeric Metafunction |60 -->
</div>
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